AC-3 is a transform-based audio coding algorithm designed to provide data-rate reduction for wide-band signals while maintaining the high quality of the original content. In the consumer electronics industry AC-3 soundtrack can be found on the latest generation of laser disc, can be found as the standard audio track on Digital Versatile Discs (DVD), is the standard audio format for High Definition Television (HDTV), and is being used for digital cable and satellite transmissions.
AC-3 allows transmission bitrate to change with each frame (approximately 32 ms.), since the bitrate information is part of the side-information bits in the AC-3 frame. In most cases, a constant bitrate is desired since it reduces software and hardware complexities thereby providing an encoding scheme suited for consumer products such as DVD and HDTV.
However, with new applications such as audio streaming over Internet and audio broadcast over mobile equipment, the constant bitrate encoder is not always the best answer.
Constant bitrate encoding schemes may have the disadvantage of providing variable quality. When a signal being compressed is psychoacoustically-simple (single tone), the encoder does a very efficient job and is able to compress it to a size much below the specified frame length (equivalently, the specified bitrate) and still maintain the coding error below the audible range. To produce a frame of the pre-defined size, it then has to perform some sort of zero padding. This may happen at times when the network is bitrate hungry. On the other hand, if this compressed data is to be archived on to a media, much space might be wasted in storing such zeros.
When the audio signal is complex (e.g. castanet), the pre-defined bitrate may not prove sufficient for the encoder. Nevertheless, to respect the constant bitrate agreement, the encoder would degrade the coding quality to the extent of producing noisy or annoying sounds.
Constant bit-rates may be the most desirable property in some applications, but for applications with more flexibility in terms of bitrate, a scheme is required to exploit this freedom for a more intelligent utilisation of bandwidth.